Frequency offset compensation for multi-carrier communication systems

Abstract

The present disclosure provides a frequency offset compensation technique for use by a multi-carrier receiver. Compared with legacy receiver designs, the technique introduces a new function which correlates various frequency offsets of the different carriers involved in multi-carrier signal transmission and reception. This new function can be coupled with an Automatic Frequency Control (AFC) function which can then utilize the correlation information provided by the new function to achieve significantly better frequency error/offset estimation when the carriers are correlated. As a method aspect, the frequency offset compensation technique comprises the steps of receiving signals modulated on a plurality of carrier frequencies, deriving a frequency offset for each carrier frequency to thereby provide a plurality of frequency offsets corresponding to the plurality of carrier frequencies; correlating these plurality of frequency offsets; and controlling compensation of the frequency offsets based on a result of the correlation.

The invention claimed is: 
     
       1. A frequency offset compensation method for a multi-carrier receiver, the method comprising:
 receiving signals on each of a plurality of carriers at respective carrier frequencies; 
 deriving a frequency offset for each carrier frequency; 
 correlating the plurality of frequency offsets corresponding to the plurality of carrier frequencies; and 
 controlling compensation of the frequency offsets based on a result of the correlation. 
 
     
     
       2. The method according to  claim 1 , wherein deriving the frequency offset for each carrier frequency comprises:
 deriving a frequency error signal based on a set of reference symbols included in the signal received on the respective carrier; and 
 accumulating such frequency error signals for each carrier with respect to one or more sets of the reference symbols received on the carrier, to obtain an accumulated frequency error signal for each carrier. 
 
     
     
       3. The method according to  claim 2 , wherein correlating the frequency offsets comprises:
 for each carrier, forming a group of the accumulated frequency error signals comprising a plurality of the accumulated frequency error signals obtained with respect to a plurality of the sets of reference symbols received on the carrier; and 
 calculating a correlation coefficient between the groups of the accumulated frequency error signals. 
 
     
     
       4. The method according to  claim 3 , wherein controlling compensation of the frequency offsets comprises comparing the correlation coefficient with at least one threshold. 
     
     
       5. The method according to  claim 4 , wherein comparing the correlation coefficient comprises determining at least one of:
 whether the correlation coefficient is greater than a first threshold; and 
 whether the correlation coefficient is less than a second threshold, wherein the first threshold is greater than the second threshold. 
 
     
     
       6. The method according to  claim 5 , wherein controlling compensation of the frequency offsets further comprises:
 computing a channel estimate for each receiving antenna-carrier-reference symbol combination; 
 computing, with respect to each receiving antenna-carrier combination, at least one phase difference between the channel estimates corresponding to subsequent reference symbols included in the set of reference symbols; 
 obtaining, for each receiving antenna-carrier combination, an average of the at least one phase difference; 
 summing the averaged phase differences over a plurality of the receiving antennas and carriers; 
 deriving a frequency correction value based on the summed average; and 
 compensating the frequency offsets using the frequency correction value derived. 
 
     
     
       7. The method according to  claim 6 , further comprising dynamically changing the number of the channel estimates used in the computation of the at least one phase difference. 
     
     
       8. The method according to  claim 6 , wherein summing the averaged phase differences comprises using different weighting factors for different carriers. 
     
     
       9. The method according to  claim 8 , wherein a higher weighting factor is used for the averaged phase difference obtained for a carrier whose frequency offset has a lower variance or standard deviation. 
     
     
       10. The method according to  claim 5 , wherein controlling compensation of the frequency offsets further comprises:
 computing a channel estimate for each receiving antenna-carrier-reference symbol combination; 
 computing, with respect to each receiving antenna-carrier combination, phase differences between the subsequent channel estimates corresponding to all the reference symbols included in the set of reference symbols; 
 summing, for at least one carrier, the phase differences over a plurality of receiving antennas and the reference symbols in the set of reference symbols to obtain a summed phase difference; 
 deriving, for the at least one carrier, a frequency correction value based on the summed phase difference; 
 obtaining a weighted average of the frequency correction values corresponding to the plurality of carriers; and 
 compensating the frequency offsets using the average frequency correction value obtained. 
 
     
     
       11. The method according to  claim 10 , wherein obtaining a weighted average of the frequency correction values comprises using different weighting factors for different carriers. 
     
     
       12. The method according to  claim 11 , wherein values of the different weighting factors depend on signal-strengths or signal-to-interference ratios of the different carriers. 
     
     
       13. A computer program product stored on a non-transitory computer-readable medium and comprising program code for a computing device in a multi-carrier receiver that is configured to receive signals on a plurality of carriers at respective carrier frequencies, said program code comprising program instructions to configure the computing device to: derive a frequency offset for each carrier frequency; correlate the plurality of frequency offsets corresponding to the plurality of carrier frequencies; and control compensation of the frequency offsets based on a result of the correlation. 
     
     
       14. An apparatus for frequency error compensation in a multi-carrier receiver that is configured to receive signals on a plurality of carriers at respective carrier frequencies, the apparatus comprising:
 a derivation unit configured to derive a frequency offset for each carrier frequency; 
 a correlator configured to correlate the plurality of frequency offsets corresponding to the plurality of carrier frequencies; and 
 a controller configured to control compensation of the frequency offsets based on a result of the correlation. 
 
     
     
       15. The apparatus according to  claim 14 , wherein the derivation unit is further configured to:
 derive, based on a set of reference symbols included in the signal received on the respective carrier, a frequency error signal; and 
 accumulate, for the respective carrier, such frequency error signals with respect to one or more of the sets of the reference symbols, to obtain an accumulated frequency error signal. 
 
     
     
       16. The apparatus according to  claim 15 , wherein the correlator is further configured to:
 for each carrier, form a group of the accumulated frequency error signals comprising a plurality of the accumulated frequency error signals obtained with respect to a plurality the sets of the reference symbols; and 
 calculate a correlation coefficient between the groups of the accumulated frequency error signals. 
 
     
     
       17. The apparatus according to  claim 16 , further comprising a comparator configured to compare the correlation coefficient with at least one threshold. 
     
     
       18. The apparatus according to  claim 17 , further comprising a frequency corrector configured to:
 compute a channel estimate for each receiving antenna-carrier-reference symbol combination; 
 compute, with respect to each receiving antenna-carrier combination, at least one phase difference between the channel estimates corresponding to subsequent reference symbols included in the set of reference symbols; 
 obtain, for each receiving antenna-carrier combination, an average of the at least one phase difference; 
 sum the averaged phase differences over a plurality of antennas and carriers; 
 derive a frequency correction value based on the summed average; and 
 compensate the frequency offsets using the frequency correction value derived. 
 
     
     
       19. The apparatus according to  claim 17 , further comprising a frequency corrector configured to:
 compute a channel estimate for each receiving antenna-carrier-reference symbol combination; 
 compute, with respect to each receiving antenna-carrier combination, phase differences between the subsequent channel estimates corresponding to all the symbols included in the reference-symbol set; 
 sum, for at least one carrier, the phase differences over a plurality of receiving antennas and the symbols in the reference-symbol set to obtain a summed phase difference; 
 derive, for the at least one carrier, a frequency correction value based on the summed phase difference; 
 obtain a weighted average of the frequency correction values corresponding to the plurality of carriers; and 
 compensate the frequency offsets using the average frequency correction value obtained. 
 
     
     
       20. A multi-carrier receiver that is configured to receive signals on a plurality of carriers at respective carrier frequencies, said multi-carrier receiver including an apparatus comprising:
 a derivation unit configured to derive a frequency offset for each carrier frequency; 
 a correlator configured to correlate the plurality of frequency offsets corresponding to the plurality of carrier frequencies; and 
 a controller configured to control compensation of the frequency offsets based on a result of the correlation. 
 
     
     
       21. A user terminal including a multi-carrier receiver that is configured to receive signals on a plurality of carriers at respective carrier frequencies, said user terminal further including an apparatus comprising:
 a derivation unit configured to derive a frequency offset for each carrier frequency; 
 a correlator configured to correlate the plurality of frequency offsets corresponding to the plurality of carrier frequencies; and 
 a controller configured to control compensation of the frequency offsets based on a result of the correlation. 
 
     
     
       22. A base station including a multi-carrier receiver that is configured to receive signals on a plurality of carriers at respective carrier frequencies, said base station further including an apparatus comprising:
 a derivation unit configured to derive a frequency offset for each carrier frequency; 
 a correlator configured to correlate the plurality of frequency offsets corresponding to the plurality of carrier frequencies; and 
 a controller configured to control compensation of the frequency offsets based on a result of the correlation.